Boulder and massive corals, like this
boulder star coral (Montastrea annularis), are the
"builders" of the reef. A coral head is a colony of small
animals called polyps. Polyps the size of a pencil eraser
build an external skeleton of calcium carbonate around their
bodies. The walls of these skeletons form a rock-like
structure. Over time, as new polyps build their skeletons on
top of each other, a large coral head is formed. The boulder
and massive coral skeletons develop the main reef structure.
Coralline algae and encrusting corals glue everything
together. (photo: William Harrington)